This disclosure relates to coating powder compositions, and more specifically coating powder compositions for deposition on substrates with low electrical conductivity.
Coating powders are dry, finely divided, free-flowing solid materials at room temperature. They are conveniently applied using electrostatic methods. In electrostatic powder coating, an electric potential is generated between the coating powder and the substrate to be coated, causing the powder particles to be attracted to the substrate. Charging of the powder may be effected by an applied voltage or by friction (tribocharging). Electrostatic powder coating has most often been used for metal substrates that are natural conductors of electricity. When substrates are non-conductive or dielectric, the substrates must be treated to make them permanent or temporary electrical conductors.
For materials such as wood, electrically charged primers have been developed. For example, U.S. Pat. No. 4,686,108 to Nason, et al. discloses applying a non-aqueous, surfactant-free primer comprising a conductive polymeric material to a wooden substrate, drying the coating, then applying a coating powder. A drawback of using a primer is that it requires the application of at least two distinct coats.
In U.S. Pat. No. 5,585,426 to Williams, et al. a process for improving the electrostatic charge developed on a coating powder composition for electrostatic coating is disclosed. The powder, which contains an essential electrostatic property-modifying agent (a polyalkylene ether, a polyalkylene glycol, a polyethoxylated stearyl alcohol, a quaternary ammonium salt or a halogenated ammonium salt) incorporated into the resin, is charged by electrical induction/conduction, and then sprayed onto a grounded solid substrate. Once sprayed, the charge facilitates the adherence of the coating powder to the substrate and enables thermal fusing of the coating powder to produce a permanent finish. A drawback of this method is that the coating is subjected to inductive or electrically conductive conditions, thus adding an extra step to the process.
U.S. Pat. No. 5,747,150 to Yamamoto et al. discloses metal substrates having multiple coating layers deposited by electrostatic methods, wherein an undercoating layer is formed from a coating powder composition comprising conductive particles such as carbon black, a conductive metal oxide such as zinc oxide or tin oxide, or an organic antistatic agent such as known anionic, cationic, or anionic surface-active agent. Use of the conductive undercoating improves transfer efficiency of a subsequent coating powder forming an upper layer. A disadvantage of this method is the use of multiple coats. Similarly, EP 260 638 to Nimura et al. discloses a coating powder composition comprising a conductivity-increasing additive such a metal complex azo compound, a phthalocyanine compound, an anthraquinone compound azine compound, an alkyl onium salt, or the metal salt of an organic carboxylic acid.
Another problem associated with the electrostatic powder coating of wood substrates is that during heating of the substrate, moisture tends to be dispelled at a greater rate from the sharp edges, corners, and intricately machined surfaces. Non-uniform moisture evaporation from the surface tends to render the electrostatic attraction of the powder to these areas difficult. Oftentimes, the surfaces of these areas are not coated at all. The accordingly remains a need for improved coating powders for low conductivity substrates.